Several recent observations in our laboratory including the demonstration of a unique methylated 5'-terminal cap structure in mRNA, the presence of a specific cap structure in hnRNA, and the changing ratio of cap structures in cytoplasmic mRNA during continuous labeling, suggest that at least two separate methylation events occur during processing and transport of mRNA in eukaryotic cells. These events may be critical in the regulation of genetic expression. The role of specific methylation in the processing of mRNA sequences will be studied in vitro in isolated murine myeloma nuclei. Radioactive mRNA will be synthesized and then transported in vitro under various conditions to study nuclear and cytoplasmic specific methylation patterns. A comparison between RNA transcribed and transported in vitro and RNA transcribed in vivo and released from isolated nuclei will be made. The in vitro RNA transport system developed from murine myeloma cells provides a unique opportunity to dissect post-transcriptional processing of RNA, thus separating nuclear from cytoplasmic events.